A family of steels with particular interest having special properties of its own is the boron steels. These steels are useful for applications having critical hardenability specifications, and advantageously exhibit uniform response to heat treatment, as well as good machinability, formability, and weldability.
The effects of boron on the hardenability of steel are similar to those obtained with such common alloying elements as manganese, chromium, nickel, and molybdenum, but, unlike these elements, only a minute amount of boron is required. Since boron is relatively plentiful in this country, in many instances it can replace the aforementioned alloying elements, many of which must be imported at considerable expense from countries where political unrest is commonplace, making at least some sources of supply uncertain.
To develop the maximum hardenability effect, boron must be present in the steel in elemental form. Since boron has a strong affinity for oxygen and nitrogen, these elements either must be removed or controlled for boron to have its full hardenability effect. Accordingly, it has been the general practice to add boron to steel with titanium and zirconium present to protect the boron from nitrogen, and aluminum to protect boron from oxygen. In addition to effecting deoxidation and providing protection of boron from oxygen, aluminum is an effective grain refiner in production of ingot cast fine-grained steel. However, aluminum or alumina residuals in the steel may be detrimental to surface quality and other desired properties in the cast steel.
In the past decade, particular interest has been given to casting of steel continuously into semi-finished shapes, such as slabs, blooms, and billets, since such procedure eliminates the ingot and primary mill stages of rolled steel production, thus providing important economic advantages.
In the continuous casting process, molten steel is poured from the ladle into a tundish containing one or more nozzles of highly refractory material, through which the molten metal flows in metered amounts to vertically extending water-cooled, open-ended molds. The solidified metal is supported and withdrawn from the molds by pairs of withdrawal rolls, following which the castings are cut into appropriate lengths.
Before being cast, the molten steel must be deoxidized sufficiently to prevent pin-holes from forming on the surface of the solidified shapes as they are cast. Such holes can weaken the thin solidified skin that is formed around the molten steel of the interior of the cast shape, increasing the opportunity for escape of molten metal therefrom and producing surface imperfections. The common deoxidizer for molten steel is aluminum.
In continuous casting of billets and blooms, the tundish nozzles usually have a diameter of less than 1.0". Aluminum-killed steels cause clogging of the tundish nozzles due to deposition of alumina therein formed during deoxidation and/or reoxidation. As the aluminum content of the steel increases above about 0.004%, the danger of tundish nozzle blockage increases, particularly in smaller diameter nozzles. Such clogging terminates the continuous casting operation, and as a result, the remainder of the heat must be cast in ingots or returned to the furnace. In either case, such procedure adds substantially to production costs.
According to U.S. Pat. No. 3,626,862, tundish nozzle clogging during continuous casting of steel can be prevented by deoxidizing the steel by addition thereto of one or more rare earth metals or compounds thereof other than the oxides. On the other hand, the literature indicates that one pound of rare earth metal per ton of steel caused blockage of tundish nozzles (7/32" diameter) in laboratory tests. See J. W. Farrell et al.: "Steel Flow Through Nozzles: Influence of Deoxidizers", Electric Furnace Proceedings, Volume 29, page 31 (1971).